Nuclear power generation and atomic bomb, which uses a solid nuclear fuel, are dependent on a fission reaction in which nuclei of uranium-235 or plutonium-239 are artificially destroyed. During this process, the nuclei of uranium-235 or plutonium-239 are split into two or more nuclear transformation product (nuclides or elements), and energy is generated at that time. A process of slowly carrying out the fission under control is power generation, while a process of releasing instantly is used for a bomb. A plurality of nuclides resulting from the fission (fission products) generally lacks a balance between the number of protons and the number of neutrons; the nuclides therefore become radionuclides having radioactivity. The form of those radionuclides (gas/liquid/solid) and the intensity of radioactivity vary according to the type of the nuclide. The radionuclides eventually become other stable nuclides with no radioactivity as the radionuclides release radiation; the time required for that process, however, varies greatly according to the type of the nuclide. The amount of time required for a nuclide to lose half the radioactivity thereof is referred to as the half-life of the nuclide. The level of radioactivity of a nuclide with a short half-life is high. Radioactive krypton and radioactive xenon, which are fission products, are gases at room temperature. Radioactive cloud mainly consisting of the above substances emit intense radiation around when moving. However, after the radioactive cloud passed and went away, there is no radioactivity left. The half-life of gaseous radioactive iodine is eight days, and therefore almost all of the gaseous radioactive iodine will disappear in a half year.
Radioactive cesium turns to gas at 678 degrees Celsius. Therefore, a nuclear accident likely would entail the release of radioactive cesium, which easily spreads widely in the environment. Moreover, the half-life of the cesium is long, i.e. 30 years. And furthermore, cesium can easily bind to soil particles, and therefore does not flow away from the earth's surface for a long time. Accordingly, cesium remains even after a radionuclide with a short half-life and radioactive iodine disappear; cesium continues to emit radiation from the earth's surface, and is absorbed into agricultural products, thereby causing long-term exposure. By the late 1960s, atmospheric nuclear tests released large quantities of fission products, or 105 million times 10 quadrillion becquerels, and contamination spread all over the globe. Radioactive cesium generated by nuclear tests still remain in the ocean, on the earth's surface, and in the atmosphere. The Chernobyl nuclear accident left a scattering of heavily contaminated areas in a range with a diameter of about 250 km. After the Fukushima nuclear accident, radioactive cesium was detected even from tea leaves in Shizuoka Prefecture, which is far away from the nuclear plant.
The half-life of radioactive strontium is 28 years, posing the same problems as radioactive cesium. However, radioactive strontium is released in atomic bomb tests or accidents at nuclear power plants, in which a reactor core is completely destroyed as in the case of the Chernobyl nuclear accident. Therefore, the spread of radioactive strontium in the environment is limited compared with radioactive cesium. Therefore, in view of radioactive-material contamination of the environment, it is very important to take measures against radioactive cesium.
A known method of removing awkward radioactive cesium, from a substance in the environment that is contaminated with it and putting together it into a particular area, is a process comprising the steps of putting, in water, a substance with a surface to which radioactive cesium in the environment is attaching, dissolving the water-soluble radioactive cesium in water, and dissolving ferrocyanide, such as iron ferrocyanide or nickel ferrocyanide, in the water, thereby allowing the radioactive cesium in the water to be adsorbed onto the ferrocyanide (Non-Patent Document 1).
According to this process, soil is dispersed in water, and radioactive cesium attaching to the surface of the soil is therefore dissolved in the water. However, this process is not sufficient. The reason, though relatively well known, is that radioactive cesium is easily captured by clay minerals in the soil. By making use of this property, even tentative attempts have been made to use clay to clean the environment contaminated with radioactive material. In other words, once radioactive cesium gets into clay, the radioactive cesium cannot be easily removed. Among various kinds of clay, if cesium is adsorbed into a mineral called illite, which is one type of mica, the cesium may not be utilized by plants and be fixed to the soil (Non-Patent Documents 2, 3); one possible reason is that cesium ions are unlikely to come out of a layer of illite as the cesium ions are caught in the layer. If cesium is unlikely to move from soil to plants, cesium is also unlikely to move to plants that people eat, helping reduce the problems. If plants have enough potassium, it is difficult for the plants to absorb cesium. It is known that, if plants do not have enough potassium, the plants can alternatively capture cesium. It is unclear whether this phenomenon is also applied to clay. If that is the case, keeping excessive amounts of potassium fertilizer at all times is actually difficult because the potassium fertilizer is expensive. Moreover under a high concentration of cesium, it is hard to do farm work. Furthermore, consumers may not feel comfortable buying farm products produced from the land where the amount of cesium is not at an inconsiderable level. In the first place, clay is an essential soil component in rice-farming soil to keep water. Even if a surface layer of the contaminated land is removed and replaced with non-contaminated soil, the problem remains as to how to deal with large quantities of contaminated soil. Therefore if cesium adsorbed into clay cannot be removed, rice farming will be significantly affected.
There is a well-known method of removing the radioactive material, from soil contaminated with so radioactive material that has a relatively long half-life and a high radioactivity level, it is a method of using gramineous plants, such as sweet sorghum, or other plants, such as sunflower, rape, pasture grass, or cabbage, to absorb the radioactive material. And then, as a subsequent process, a process of burying the plants in the earth or incinerating the plants is carried out. However, if the plants themselves are buried in the ground, a large area of land and enormous labors are required. In the case of the incineration process, the radioactive material adsorbed into the cells of the plants becomes condensed during the incineration process, and the high-concentration radioactive material might be released into the atmosphere as a gas even after passing through a filter.
Another known method is to carry out a fermentation process of plants and turn the resultant organic substances into biofuel (Non-Patent Document 4).
However, the disadvantages with this method are that, in the case of lignin or cellulose, the fermentation process takes a lot of time because the molecular weight thereof is high, and that the radioactive material still contained in the cell membrane cannot be removed. Moreover, the method is totally ineffective for the soil.
Still another known method is to use algae “binos”: radioactive material is absorbed into cells of the algae from water contaminated with the radioactive material (Non-Patent Document 5). However, this method is only used for capturing the radioactive material dissolved in the water, and cannot be used for taking out the radioactive material in the body of an organism as in the case of the above-described method. Therefore the problem remains to how to deal with the radioactive material absorbed into the algae.
In the aftermath caused by an accident at a nuclear power plant or the like, large amounts of pollutants are generates. if the level of radioactive contamination thereof is not so high, a method of incinerating the pollutants is employed. After the 2011 Fukushima nuclear accident, large amounts of pollutants were incinerated, judging that the level of contamination thereof was not so high.
However, the radioactive material in incineration ash can be concentrated to, for example as much as 50,000 Bq/kq in terms of radioactivity level. Usually, in order to prevent the generation of dioxin, the incineration ash with porous media having such a fine structure that are high in water absorbability is exposed to about 900 degrees Celsius. The radioactive material is certain to be stored in pores of the porous media.
There is no specific report that the radioactive material has been taken out of incineration ash and reduced to a safe level in the living environment. Therefore there is a problem that large quantities of incineration ash are left unprocessed.